Soil Fumigant and Herbicide Combinations
for Soilborne Pest Control in Caladium

James P. Gilreath, Robert McSorley, and Robert J. McGovern, University of Florida, Gulf Coast Research and Education Center, 5007 60th Street East, Bradenton, FL 34203 (jpg@gnv.ifas.ufl.edu)

Greater than 95 percent of the world's production of caladium tubers occurs in a small geographical area near Lake Placid, Florida. Most of the production is on muck or high organic matter soils. Soilborne pest control is a major problem for caladium producers with pests ranging from weeds to root-knot nematodes and soilborne diseases, such as Fusarium. Although most caladium growers fumigate their soil with methyl bromide, results of research over the years have not demonstrated a clear response to soil fumigation under all conditions. Growers routinely soak their tubers in hot water as a means of killing root-knot nematodes and some superficial disease inoculum, but it has only been where root-knot nematode populations in the soil were high that a response to soil fumigation was demonstrated in previous research in muck soil. Few caladiums are produced in mineral soils (fine sand) in Florida, but for those which are grown in these soils, a very positive response to soil fumigation has been demonstrated. In none of this early research was there an effect on soilborne disease control in the tubers at harvest as a result of fumigation, especially with regard to Fusarium sp. which infected all of the tubers in the few previous studies, whether grown in fumigated or nonfumigated soil, suggesting that the source of this disease was the planting stock itself. Various fumigants have been evaluated over the years, including mixtures of dichloropropene, dichloropropane and methyl isothiocyanate (Vorlex), 1,3-dichloropropene + 1,2-dichloropropane (D-D), and 1,3-dichloropropene + chloropicrin (Telone C-17 or C-35), as well as metam, but none are believed to be as effective as methyl bromide/chloropicrin mixtures for control of all the pest spectrum.

Although methyl bromide is generally thought to be very effective against most weed propagules, weeds often are a problem in caladiums due to the long growing season (8 to 9 months). For many years growers relied upon alachlor for weed control, but it was withdrawn from sale in Florida in the mid 1980's, leaving growers with no reliable substitute. Research during the 1980's demonstrated that oryzalin could be used successfully for weed control in caladium in both mineral and organic soils and later metolachlor also was demonstrated to be an effective herbicide for use in caladiums. As a result, growers have developed weed control programs using these herbicides and selective hand weeding, but the programs almost always are used in combination with methyl bromide due to the multiple pest complex generally encountered during tuber production.

Historically, growers have relied upon soil fumigation with methyl bromide to control soilborne pests; however, the impending phaseout of methyl bromide will eliminate that practice. To find a suitable alternative to methyl bromide as a soil fumigant in caladium tuber production, a two-year study was initiated on a commercial farm near Lake Placid, FL, beginning with the 1998 crop season, to assess the efficacy of selected soilborne pest control programs consisting of combinations of soil fumigants and herbicides.

The test area was located on a sandy muck soil south of Lake Placid, FL, and, like most commercial tuber farms in the area, the test area had been fumigated with methyl bromide for several years prior to test initiation. Treatments were applied to 50 ft long by 22 ft wide plots which were arranged in a randomized complete block design and replicated 6 times. Each plot contained 5 beds, consisting of 4 rows each. Fumigant treatments evaluated each of the two years consisted of 1) no fumigant; 2) methyl bromide/chloropicrin (90/10 percent) at 450 lbs./acre; 3) 1,3-dichloropropene (1,3-D)/chloropicrin (83/17 percent) (Telone C-17) at 35 gal./acre; and 4) 75 gal. of metam sodium (Vapam) per acre + 200 lbs. of chloropicrin (pic) per acre. Metolachlor herbicide (8 lbs./acre) was applied at planting to plots treated with 1,3-D or metam, in 1998. Oryzalin (4 lb./acre) was applied at planting during 1999. All fumigant treated plots, including the methyl bromide plots, received an over the top application of oryzalin (4 lb./acre) 7 weeks after planting in both 1998 and 1999. The nontreated control received no fumigant or herbicide during the course of this experiment.

Data collected at appropriate intervals included crop plant vigor, weed control by species, numbers of nematodes in the soil, plant disease observations and tuber production. Weed control was evaluated five times during the season and all plots were hand-weeded after each evaluation. The predominant weed species present were Digitaria ciliaris (Retz.) Koel. (crabgrass), Amaranthus viridis L. (pigweed) and Portulaca oleracea L. (purslane). Caladium tubers were dug from the center bed of each plot in January and February of the following year with a commercial digger, collected into crates, and transported to a caladium barn where they were washed free of soil and debris and stored in ventilated crates to dry. Approximately two weeks later all tubers were sorted and hand graded into the industry-standard commercial size grades. A production index assessment which provides a numerical scale to relate tuber production to the number of number one size tubers also was determined.

Plant vigor in 1998 was higher in methyl bromide treated plots in midsummer, but by mid-fall plants in areas treated with 1,3-D + chloropicrin were just as vigorous as those where methyl bromide had been applied. During 1999, caladiums grown in 1,3-D + chloropicrin treated soil appeared to be the most vigorous, but were no more vigorous than those grown in methyl bromide fumigated soil. Early control of weeds was good with those treatments which received herbicide at time of planting (metolachlor in 1998 and oryzalin in 1999), but methyl bromide allowed an early infestation of crabgrass and pigweed, indicating that it would have benefitted from herbicide application at planting and suggesting that loss of methyl bromide from the upper 2 inches of the soil may have been too rapid for good weed control in this test. Subsequent evaluations of weed control indicated that application of herbicide (metolachlor or oryzalin) at planting followed by an over-the-top application of oryzalin approximately 7 weeks later, all combined with occasional hand weeding, provided acceptable weed control until late fall when caladium growth was slowing and growers generally are not concerned with weed control. The choice of soil fumigant had little to no effect on weed control when combined with herbicide. The impact of an effective herbicide program was seen in the total number of weeds counted in the plots over the season as there were 10 to 20 times as many weeds present where herbicide was not applied than where it was used, in spite of multiple hand weedings. By the time of harvest in January and February, 72 percent of the soil surface was covered with weed growth in the nontreated plots whereas only about 20 percent of the soil surface was covered in plots which received herbicides as well as fumigants.

Nematodes were not a factor in this study. Even though each treatment plot was located in the same spot each of the two years, few nematodes were found in soil or tissue samples. Fusarium was detected in tubers prior to planting at an incidence of 100 percent in 1998 and slightly less in 1999, whereas Pythium was not detected. There were no significant differences among treatments for incidence of any disease organism in either year of the study. When the tubers were inspected and sampled for disease organisms after harvest, Erwinia, Fusarium, Pythium and Rhizoctonia were found in most of the tubers. Soil fumigation had no effect on incidence of these organisms in the tubers at harvest.

Tuber production was determined for each treatment after digging, washing and drying the tubers in January 1999 and February 2000. There was no difference in production for any size grade, except jumbo, where significantly more tubers were produced in plots treated with 1,3-D + chloropicrin with metolachlor at planting followed by oryzalin in mid summer in 1999. There were no differences in jumbo production between methyl bromide or metham + chloropicrin or where no fumigant or herbicide was applied in the first year of this study. During the second year of the study, there were no differences in production of any size grade of caladium tuber with the fumigant/herbicide combinations. Yield was as good with no fumigant or herbicide applied as it was where fumigant and herbicide were used. Cost of production was different because labor for hand weeding was a major expense for the nontreated control plots. Thus, although there was not a direct effect on tuber yield of the fumigant/herbicide option chosen, there was an indirect effect on the economics of production.

Previous use of methyl bromide on this field may have influenced results by reducing pest levels below what might have been observed in an area which had not been subjected to repeated applications of methyl bromide. Many scientists expect this effect to exist and carry forward for several years into the future once methyl bromide use ceases. Caladium growers have reported that they can count on only one year of grace past the season in which they use methyl bromide. In the past when they have used other fumigants in subsequent years, the first year of an alternative has been successful, but if they try for a second year they usually suffer reduced yields and poorer soilborne pest control. Lack of more significant differences in pest levels and tuber production in this study suggest 1) the levels of soilborne pests were moderate to low in this field at the time of fumigant application, 2) tubers already contained some pests and this masked treatment differences, or 3) soil fumigation or the fumigants tested are not useful under the current production system. Caladium tubers were hot water treated prior to planting to kill root-knot nematodes within the tissue, thus nematodes would be observed only if they were present in the soil. Since nematodes were not recovered from soil samples during the season or tubers at the time of harvest, it is apparent that nematodes were not present in the test area. We observed a slight response to soil fumigation in the form of an increase in production of jumbo size tubers with 1,3-D + chloropicrin and herbicide during 1998 but none in 1999. Previous research, as well as the current study, demonstrate that planting tubers already infected with Fusarium prior to planting guarantees Fusarium as a pest in caladiums, regardless of soil fumigation.

Results of this research suggest that 1,3-D + chloropicrin (83/17 percent, Telone C-17) at 35 gal/acre may be a viable replacement for methyl bromide when combined with metolachlor or oryzalin herbicide at planting followed by a midsummer application of oryzalin. Furthermore, it was observed that even methyl bromide would have benefitted from application of herbicide at planting to control early weed emergence and growth, especially that of crabgrass and pigweed. This work is being continued in an effort to determine the long term effects of these fumigant/herbicide combinations on pest control and tuber production.

Last Updated: November 22, 2000